feat(core): Expose SCSS breakpoints

### Feature Description

As @angular/flex-layout become deprecated and Angular team does not intend to provide an alternative, we are forced to handle responsive layout by our own.

Writing CSS is ok, but to stick with Material design guidance we must use specific breakpoints. It would be great if @angular/material could expose these breakpoints like it does for palettes and theming. Otherwise we have to declare them ourself for every projects.

### Use Case

would simplify scss writting and add opinions about breakpoints (like CDK Layout API)

```scss
@use '@angular/material' as mat;

@media #{$mat.gt-xs} {
// insert your styles
}
```

首先，将请求的 SCSS 断点变量与 issue 中提到的 CDK Layout API 和 Angular Material 主题化接口进行比较。当 Angular Material 提供有文档说明的断点值，并且这些值可以在 SCSS 媒体查询中使用而无需在每个项目中重新定义时，即表示完成。

由索引模型根据 Issue 内容生成。